The present invention concerns a bracelet made of articulated elements, in particular links at least some of which are preferably produced at least in part in a material of low impact resistance. The invention is also directed to a particular use of such a bracelet for watches, jewelry, or even ornaments.
The incorporation in bracelets of materials that are hard and generally have a low impact resistance, namely materials sensitive to mechanical shock, is essentially intended to confer great durability (resistance to scratching and wear) and esthetic added value to the object to which they are fitted. The latter generally consists of a metallic armature that is needed in order to be able to resist high accidental mechanical loads, such as those that occur if it is dropped.
In contrast to metals and alloys obtained by casting, materials such as those produced by solidification at high temperature of a paste or compressed powder generally have a low impact resistance and are therefore particularly sensitive to mechanical shocks. This sensitivity is the result of the absence of plastic deformation of these materials on impact. This means that parts constructed from such materials suffer what is called brittle fracture.
Materials defined as fragile under normal temperature conditions and at low impact speeds include for example sintered hard metals, all types of ceramic including zirconia, glass, and minerals such as sapphire and ruby.
The documents EP 586 981 and EP 347 841 describe bracelets consisting of links, notably for watchstraps, the visible parts of which are formed of decorative elements produced in hard materials, such as ceramics or sintered hard metals. The bracelet includes an armature consisting of link assemblies in a material that can be machined, such as steel. In both cases, ceramic elements are intended only to cover the links of the armature, which is what withstands all mechanical forces applied to the bracelet. Thus the mechanical stresses to which the decorative elements are subjected are greatly limited.
The document US 2002/0009019 suggests another bracelet construction formed of a succession of ceramic links each forming an entity constituted of a central link bordered by two lateral links offset relative to the former. These three links are held together by a pin threaded at its ends, which are intended to be screwed into threads molded into the lateral links. The articulation and assembly of the successive link assemblies are obtained by a spring bar passing through the central link and having an elastic member housed in its central part. This member is intended to push apart two tenons slidably mounted at the ends of the bar so that they can be inserted into housings formed in corresponding relationship in the lateral links.
The principal drawback of such a construction stems from the fact that this kind of bracelet suffers from breakage of one or more ceramic links, as it has been possible to confirm during tests reproducing accidental dropping of a watch attached to this type of bracelet. On such dropping, the mass of the watch case plus the watch movement generates large forces on the pins or bars of the bracelet, principally because of a lever arm effect between the point of impact of the bracelet on the floor and the fixing of the bracelet to the watch middle. The break is almost systematically located at the level of the articulation pins of the links. Simulations have shown that high stresses are concentrated in the corners at the entrance to the holes in the ceramic links. It has also been found that all breaks occurred in the direct vicinity of the pins, at the edges of the ceramic links. A broken link or link assembly of such a bracelet renders it unusable and generates non-negligible repair costs through necessitating replacement of the broken element.